You have to bid $1 which will keep the bid at $15 until someone raises it. If you're willing to bid $5 then bid $20 as your max bid. AB will bid for you up to $20 if someone was to bid against you. If no one bids against you the bid will stay at $15 and that is what you'll pay. If the seller is offering by-it-now and you really want the fish then I would take that option. You may pay more but at least you'll get the fish you want. Good luck